1. Field of the Invention
The present invention relates generally to the field of fuel cells and, more specifically, to a direct methanol fuel cell system in which the need to store water in the system or fuel supply is minimized or eliminated through the provision of a water generator.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel-processing is complex, expensive and requires significant volume, reformer based systems are presently limited to comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically-conductive, but electronically non-conductive membrane (PCM). Typically, a catalyst which enables direct oxidation of the fuel on the anode is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). Diffusion layers are typically in contact with each of the catalyzed anode and cathode faces of the PCM to facilitate the introduction of reactants and removal of products of the reaction from the PCM, and also serve to conduct electrons. Protons (from hydrogen found in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and oxygen molecules involved in the cathodic reaction and travel through a load, providing electrical power.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, methanol in an aqueous solution is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two fundamental half reactions that occur in a DMFC which allow a DMFC system to provide electricity to power consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate (more specifically, failure to oxidize the fuel mixture will limit the cathodic generation of water, and vice versa).
Fuel cells and fuel cell systems have been the subject of intensified recent development because of their ability to efficiently convert the energy in carbonaceous fuels into electric power while emitting comparatively low levels of environmentally harmful substances. The adaptation of fuel cell systems to mobile uses, however, is not straight-forward because of the technical difficulties associated with reforming most carbonaceous fuels in a simple, cost effective manner, and within acceptable form factors and volume limits. Further, a safe and efficient storage means for substantially pure hydrogen (which is a gas under the relevant operating conditions), presents a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy adsorption matrices in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol or an aqueous methanol solution (and to some extent, ethanol, and other carbonaceous fluids or aqueous solutions thereof).
In particular, DMFCs are being developed for commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell, and the components may be fabricated using materials that not only optimize the electricity-generating reactions, but which are also cost effective, and allow the fuel cell system to fit demanding form factors. Furthermore, the manufacturing process associated with those materials should not be prohibitive in terms of labor intensity cost.
Typical DMFC systems include a fuel source, fluid and effluent management systems, and a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, and a membrane electrode assembly (“MEA”) disposed within the housing.
A typical MEA includes a centrally disposed protonically conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is Nafion® a registered trademark of E.I. Dupont de Nours and Company, a cation exchange membrane comprised of perfluorosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layers function to evenly distribute the liquid fuel mixture across the catalyzed anode face of the PCM, or the gaseous oxygen from air or other source across the catalyzed cathode face of the PCM. In addition, flow field plates are often placed on the aspect of each diffusion layer that is not in contact with the catalyst-coated PCM. The flow field plates may function to provide mass transport of the reactants and by products of the electrochemical reactions and also have a current collection functionality to collect and conduct electrons through the load.
The direct oxidation fuel cell based on oxidation of methanol requires water and methanol to be present together at the anode catalyst in order for the oxidation half reaction of methanol to proceed to completion. However, in an energy conversion device based upon DMFC technology, for a given energy content, the size of the device is readily reduced by carrying only methanol in the fuel reservoir as opposed to a methanol-water solution. In some architectures, a portion of the water required will be present in the anode chamber, or may be recirculated from the cathode aspect of the DMFC, however, it may be necessary to generate additional water under some conditions. In other words, the volumetric energy density of the device can be maximized by reducing the amount of water stored in the methanol-water solution within the DMFC. This would require a method of water production within the device in order to drive the anodic half reaction to completion.
It is thus an object of the invention to provide a water generator that provides water for diluting the methanol supplied to the DMFC while increasing the volumetric energy returns of the fuel cell system over its lifetime.